There are two main types of lithium storage battery: lithium-metal batteries, where the negative electrode is made of lithium metal (which material causes problems with safety when in the presence of a liquid electrolyte), and lithium-ion batteries, where the lithium remains in the ionic state.
Lithium-ion batteries consist of at least two conductive Coulombic electrodes of different polarities, the negative electrode or anode (generally made of graphite) and the positive electrode or cathode (generally made of an oxide of a transition metal, such as an oxide of vanadium or cobalt, or made of a lithiated iron phosphate such as, for example, described in documents U.S. Pat. No. 6,514,640 or WO-A1-2011/092283), between which electrodes a separator is located, which separator consists of an electrical insulator imbibed with an aprotic electrolyte based on Li+ cations ensuring the ionic conductivity. The electrolytes used in these lithium-ion batteries usually consist of a lithium salt, for example of formula LiPF6, LiAsF6, LiCF3SO3 or LiClO4, which is dissolved in a mixture of non-aqueous solvents such as acetonitrile, tetrahydrofuran, or more often a carbonate, for example of ethylene or propylene.
The active material of the cathode of a lithium-ion battery allows reversible insertion/removal of lithium into/from this cathode, and the higher the mass fraction of this active material in the cathode, the higher its capacity. The cathode must also contain an electrically conductive compound, such as carbon black and, in order to provide it with sufficient mechanical cohesion, a polymer binder. A lithium-ion battery is thus based on the reversible exchange of lithium ions between the anode and the cathode during the charging and discharging of the battery, and, for a very low weight, by virtue of the physical properties of lithium, such a battery has a high energy density.
The cathodes of lithium-ion batteries are most often manufactured using a process comprising, in succession, a step of dissolving or dispersing the various ingredients of the cathode in a solvent, a step of spreading the obtained solution or dispersion on a metallic current collector, and then lastly a step of evaporating this solvent. Many types of polymer binders can be used, among which mention may firstly be made of PVDF (polyvinylidene fluoride), which is more easily compatible with a cathode operating at a high operating voltage (more than 4 V) because of the presence of fluorine, but also, for example, polyacrylonitriles (PAN) with polybutylacrylate latexes.
Processes for manufacturing the cathodes of lithium-ion batteries that use an organic solvent have many drawbacks with respect to the environment and safety. In particular, it is, in this case, necessary to evaporate large amounts of such solvents, which are toxic or inflammable.
As for processes that use an aqueous solvent to manufacture these cathodes, their major drawback is that the cathode must be very thoroughly dried before they can be used, traces of water being known to limit the useful lifetime of lithium storage batteries.
It is therefore highly desirable, for lithium-ion batteries, to prepare cathodes that are manufactured without using solvents. It is in this context that processes for manufacturing cathodes for lithium-ion batteries using melt processing techniques (for example extrusion) have been described in the literature.
Unfortunately, these melt processes cause major difficulties in the case of lithium-ion batteries, which, as is known, require a mass fraction of active material in the polymer mixture of the cathode of at least 90% for the latter to have sufficient capacity in the lithium-ion battery. However, at such contents of active material the viscosity of the cathode polymer mixture becomes very high, and leads to a risk of the mixture over-heating or losing its mechanical cohesion once it is in use.
Document U.S. Pat. No. 6,939,383 describes the extrusion of a polymer composition comprising a poly(ethylene oxide)-poly(propylene oxide)-poly(glycidyl ether) copolymer for a ionically conducting polymer for solventless implementation of a cathode for a lithium-polymer battery. However, the mass fraction of active material in the single cathode polymer composition manufactured in this document is only 64.5%.
Document U.S. Pat. No. 5,749,927 discloses a process for the continuous preparation of lithium-polymer batteries by extrusion, which process comprises compounding the active material with an electrical conductor and a solid electrolyte composition comprising a polymer, a lithium salt and a propylene carbonate/ethylene carbonate mixture greatly in excess over this polymer. In this document, the mass fraction of active material present in the cathode polymer composition is also below 70%.
Thus, a major drawback of these known melt processes for manufacturing cathodes for lithium storage batteries is that the mass fractions of active material in the cathode polymer composition are insufficient to obtain high-performance cathodes specifically for lithium-ion batteries.